1. Field of the Invention
The present invention relates to a monocrystal growing apparatus and more particularly to a monocrystal growing apparatus building up a monocrystal rod by floating zone method.
2. Description of the Related Art
In accordance with floating zone method, an upper vertical shaft supports a polycrystal rod, a lower vertical shaft supports a seed crystal and a high-frequency coil connects both contacting tips of the polycrystal rod and the seed crystal by melting them. Then, the polycrystal rod and the seed crystal are rotated while the upper and lower vertical shafts are together descended, and a melting zone (i.e., melt) produced by the high-frequency coil is progressively transferred toward the polycrystal rod adjacent and above the melt, and thereby a monocrystal is grown on the seed crystal.
FIG. 4 illustrates this prior-art monocrystal growing apparatus. A chamber-defining housing is indicated at 1. The housing 1 has an upper vertical shaft 1a and a lower vertical shaft 1b which extend coaxially. The upper vertical shaft 1a coaxially supports a polycrystal rod 2a and the lower vertical shaft 1b coaxially supports a seed crystal 2b so that the polycrystal rod 2a and the seed crystal 2b are coaxially aligned. The housing 1 contains a dished single-turn high-frequency coil 3 placed essentially at the center of the chamber of the housing 1.
The respective upper and lower vertical shafts 1a and 1b of the prior-art monocrystal growing apparatus rotate the polycrystal rod 2a and the seed crystal 2b at equal or different rotational speeds and concurrently descend at an equal speed to progressively axially transfer the melt toward the polycrystal rod 2a adjacent and above the melt so as to build up a monocrystal rod 2c on the seed crystal 2b.
The housing 1 of the monocrystal growing apparatus receives a doping gas (e.g., B.sub.2 H.sub.6 when the monocrystal rod 2c is p-type and on the other hand, PH.sub.3 when the monocrystal rod 2c is n-type) introduced through the bottom end of the housing 1 in order to build up the p- or n-type monocrystal rod 2c of a predetermined resistivity. The monocrystal growing apparatus discharges a waste doping gas through the top of the housing 1.
The prior-art monocrystal growing apparatus has the following drawbacks in producing the p- or n-type monocrystal rod 2c:
That is, since the polycrystal rod 2a and monocrystal rod 2c (hereinafter, the two rods are commonly referred to as mere a semiconductor rod or semiconductor rods) are exposed to the atmosphere of the doping gas introduced into the housing 1, the impurity of the doping gas is not only dissolved directly into the melt but also decomposed to deposit on the high-temperature cylindrical surfaces of the semiconductor rods 2a adjacent the melt, so that the impurity is mixed and diffuses into the semiconductor rods in the course of melting of the semiconductor rods in a monocrystal growth step.
Thus, the resistivity of the monocrystal rod 2c depends on not only the amount of the impurity taken directly into the melt but also the amount of the impurity deposited on a part of the polycrystal rod 2a adjacent the melt and then directly or with a precedent diffusion dissolved in the melt, so that the resistivity of the monocrystal rod 2c is difficult to control.
That is, since the polycrystal rod 2a is progressively melted from the bottom towards the top, the polycrystal rod 2a receives the doping impurity at a progressively greater rate as the polycrystal rod 2a descends to melt because an upper part of the polycrystal rod 2a is longer exposed to the doping gas over the surface than a lower part of the same. The prior-art monocrystal growing apparatus entails a problem in that the product resistivity of the monocrystal rod 2c progressively decreases towards the tail of the monocrystal rod 2c as seen in FIG. 5.
The present invention was made in view of the above problem.